The invention relates to an amplifier arrangement for amplifying an amplitude-varying input signal and for reducing an unwanted d.c. offset thereof, comprising a threshold circuit and an amplifier stage coupled thereto, and to a directly mixing synchronous AM-receiver having an RF input, which is coupled at one end to a synchronous AM detector and at the other end to a phase-locked loop for generating a local carrier which is phase-coupled with the carrier of the RF reception signal, said phase-locked loop comprising a phase detector, a loop filter and a voltage-controlled oscillator successively arranged in a loop configuration.
An amplifier arrangement of this type is known from U.S. Pat. No. 4,277,695. A directly mixing & synchronous AM receiver of the type described above is known from British patent specification No. 2,130,826.
The threshold circuit of the known amplifier arrangement is adjustable and is used to adjust that part of the input signal for which there is no amplification. A correctly chosen adjustment of this so-called dead zone provides the possibility of suppressing unwanted d.c. offsets which may have been cause for example, by noise and parasitic d.c. disturbances, and only the desired amplitude varying signal component is amplified.
In the known amplifier arrangement, however, the desired signal component is non-linearly amplified and for a correct adjustment of the dead zone the amplitude of the unwanted d.c. offset to be reduced should be known in advance.
In practice this is not always the case and the unwanted d.c offset of the input signal to be amplified may be greatly dependent on the form in which the relevant circuit arrangement is realized, or may be influenced by the signal to noise level of the input signal or by ambient factors, or is difficult to distinguish from a desired signal component whose amplitude variation is only small relative to the d.c. offset, as may occur for example, in the phase control signal of a phase-locked loop.